A team of geophysicists has produced the most detailed three-dimensional map of the ocean floor so far by using ship soundings to correct new and recently declassified satellite data. The 68-million pixel panorama, described in today's issue of Science*, could help improve models of the ocean circulation and the sea-floor spreading process that takes place along midocean ridges.
Combining satellite gravity data with ship depth soundings reveals the plate tectonic fabric of the sea floor in detail. Depths and heights range from 7 km below sea level (dark purple) to over 6 km above it (red to lavender) on the surrounding continents. Variations in the increase in depth away from mid-ocean ridges (greens) suggest a complex pattern of heat loss from Earth.
Traditionally, researchers have mapped the ocean floor from a ship by bouncing sound waves off the bottom. Unfortunately, there are huge areas unmapped between ship tracks. A satellite, by contrast, covers a wide swath, but cannot detect the bottom of the ocean at all. Instead, it relies on the gravity of undersea ridges and mountains to pile up water into low, broad swells, which the satellite senses by bouncing microwaves off the sea surface. But satellite data can't reveal features smaller than about 12 kilometers across, and local variations in the density of the ocean floor can produce gravity anomalies mimicking those produced by seamounts (underwater volcanoes).
To address these problems, geophysicists Walter Smith and David Sandwell of the National Oceanic and Atmospheric Administration in Silver Spring, Maryland, calibrated satellite measurements against ship measurements wherever possible. "We twisted arms all over the international community to get data," Smith says. They also acquired new data from the European Space Agency's ERS-1 satellite and formerly classified data from Geosat, a U.S. Navy satellite. When they knew both the ship depth soundings and the satellite gravity measurements at a certain place in the ocean, they constructed a mathematical "transfer function" to convert gravity data into topography. They could then apply the same transfer function to the satellite gravity data over nearby regions that had not been covered by ship. The result was a "predicted bathymetry" for the whole region, with a resolution as fine as 1.1 kilometers at high latitudes--about twice the resolution of the best previous global map.
"The map looks stunning and does a great job of pinpointing the location and trends of underwater features," says Andrew Goodwillie, a geophysicist at the Scripps Institution of Oceanography in La Jolla, California. But he and other experts in ocean-floor topography warn that because the new map relies so heavily on satellite data, it should not be interpreted too literally.